1. Field of the Invention
This invention relates to distributed lighting systems, particularly for vehicles, aircraft, rooms, streets, highways and medical applications, in which optical fibers distribute light from a central source to a number of different optical loads, which can include an edge-lit display panel.
2. Description of the Related Art
Conventional automobile lighting systems use separate light bulbs for each lighting function, resulting in some cases in more than one hundred different light bulbs. This is an undesirable situation in terms of energy efficiency and reliability. Furthermore, some of the bulbs are typically placed in inconvenient locations, making them difficult to access and increasing maintenance costs. The bulbs are powered by an electrical wiring network, leading to the possibility of electrical short circuits that are often difficult to locate. The light bulb systems are also subject to breakage in case of impacts, and add to the weight, bulk and expense of the overall vehicle.
An alternate system, in which light is distributed from a central light source to various optical loads within a vehicle by means of "light busses", its described in U.S. Pat. No. 4,930,049 to Davenport et al. Lighting is provided by this network to the headlight, tail lights and interior lights of the vehicle via optical fibers from a single light source. Details of the light source are given in U.S. Pat. No. 4,958,263 to Davenport et al. It consists of a pressurized lamp with quartz light guides that are merged into portions of its outer surface to provide illumination for the various optical loads within the vehicle. The portions of the lamp that are not merged with the light guides are coated with a diffusive reflective coating that is said to substantially prevent light from being transmitted through the coating, thereby directing all of the light generated by the lamp into the light guides.
While the system described in these patents resolves many of the problems discussed above in connection with multi-bulb systems, it still has some undesirable limitations. To substitute one light source for another, either to replace a lamp that has burned out or to substitute a lamp with a different power rating, it is necessary to remove the light guides along with the lamp, and then to individually couple each of the light guides for the new lamp into the system. Since the light guides are merged directly into the lamp body, there is a direct thermal connection between the two that both adds to the heating of the light guides, and disturbs the temperature distribution of the lamp. The latter effect can impair the quality of light delivered by the system, since the lamp's spectral output is highly sensitive to temperature. An additional energy input would also be required to compensate for heat losses through the light guides, since metal halide lamps of the type employed in the cited patents require a minimum temperature of approximately 800.degree.-900.degree. C. to maintain the metal halide in its vapor phase; and no mechanism is provided for dissipating heat that has been radiated out from the lamp. Furthermore, the numerical aperture (N.A.) of the output light is much larger than the N.A. of the convention optical fibers. The coupling efficiency could therefore be substantially lower than if the beam profile between the quartz rods and the optical fibers were matched.
An automobile's lighting system will typically include one or more display panels that can function as brake lights, tail lights, dome lights and the like. One such display device is disclosed in U.S. Pat. No. 4,989,956 to Wu et al., assigned to Hughes Aircraft Company, the assignee of the present invention. In this device a fluorescent dye-doped panel is side-lit by ambient light, or by a fluorescent light source with a semi-cylindrical reflector that directs light from the source onto the panel. The panel responds by emitting light of a longer wavelength out through one edge, where it is coupled into the edge of a display panel. Light is re-emitted out from the side of the display panel by means of a distortion on its side surface which redirects light through total internal reflection. In another display device, described in application Ser. No. 07/725,216, filed Jun. 26, 1991 by Wu et al., and also assigned to Hughes Aircraft Company, a display panel is directly edge-lit by an elongated tungsten-halogen lamp whose emission is focused onto the panel edge by an elliptical reflector. Grooves that are cut into one side surface of the panel redirect the light traveling through the panel out of the opposite surface by total internal reflection; the depths of the grooves are graded to provide an even distribution of light reflected out of the panel, thereby compensating for the progressive loss of light within the panel away from its light receiving edge.
Another form of edge-lit display panel, described in U.S. Pat. No. 4,811,507 to Blanchet, employs elongated lamps at the opposite edges of the panel. Light from the edges is redirected out of the panel either by means of striations whose depths and frequency increase in a direction away from the light sources, or by "light re-emitting elements" in the form of inserts such as microballs or micro-bubbles. The inserts are distributed within the panel so that their density increases in a direction away from the light sources, and their light re-emitting power also increases in that direction. This characteristic is said to make it possible to obtain a uniform distribution of light from the panel's front surface.
Another display panel, disclosed in U.S. Pat. No. 5,036,435 to Tokuda et al., employs a sheet of multiple side-by-side optical fibers as the panel. The fibers are illuminated at one end by a discrete light source, and at their opposite ends include "flaws" in the form of notches that are cut into one side of the fiber sheet, causing light to "leak" out of the sheet. The width, density and depth of the "flaws" are preferably increased in direct portion to their distance from the light-receiving ends of the fibers to obtain a uniformity in the brightness of the emitted light.
The first three display panels described above use discrete light sources, and are thus not compatible with a system that relies upon light distribution to numerous different optical loads from a central source. While the light source in the '435 patent is located a distance away from the panel itself and is connected to the panel by means of optical fibers, the fibers also form the display panel itself. This requires multiple fibers running between the panel and the light source, and calls for replacement of both the display and the optical supply portions of the fibers if there is a need to replace either one.